Add a `linear_fake_merge` option (defaulting to True)

[cgwalters]

One of my projects (projectatomic/rpm-ostree) has a protected master branch. I was rather horrified to realize that homu force-pushes to master by default. Admittedly, I understand this is the only way to make github think the PR was merged (as opposed to just closed).

But in my mind, that's a deficiency of github - it should have an API to allow us to "close merged with this commit".

Anyways, add an option so that those who prefer closing-as-unmerged over force pushing can do that today.

[cgwalters]

Thinking about this a bit more, one thing we can do is change the title of the PR to say [merged] foo bar baz.

[cgwalters]

Now updated to also retitle the commit with [merged]

[ashcrow]

:+1: looks good!